This invention relates in general to semiconductor devices, and in particular to backward diodes useful in radio frequency detection and mixing.
The tunnel diode is a well-known semiconductor device that conventionally includes two regions of heavily doped semiconductor material of opposite conductivity types, separated by a relatively thin junction which permits charge carriers to tunnel through upon the application of a suitable operating potential to the semiconductor regions. The p and n regions of tunnel diodes are so heavily doped that they are degenerate. At equilibrium, a portion of the valence band in the p region of the diode is empty and a portion of the conduction band in the n region is filled.
A slight forward bias brings some levels of the filled portion of the conduction band of the n region into energetic alignment with empty levels of the valence band of the p region. In this situation, quantum-mechanical tunneling allows electrons to flow from the n region to the p region, giving a positive current that first increases with increasing bias. When the filled part of the conduction band of the n region is maximally aligned with the empty part of the valence band of the p region, the current flow is maximized. Subsequently, the current decreases with increasing forward bias, and approaches a minimum value when the filled part of the conduction band of the n region lies opposite the energy gap of the p region. When a yet larger forward bias occurs, electrons and holes are injected over the barrier between the p and n regions, resulting in a rapid increase in current for increasing forward bias. Thus, the current-voltage has a negative differential conductance part in the forward region of the characteristic.
Use of a heterostructure consisting of adjoining regions of GaSb1xe2x88x92yAsy and In1xe2x88x92xGaxAs interfaced with a tunneling junction is described in U.S. Pat. No. 4,198,644 entitled, xe2x80x9cTunnel Diodexe2x80x9d issued to Leo Esaki on Apr. 15, 1980. The heterostructure presented in the Esaki patent discloses first and second layers of Group III-V compound semiconductor alloys wherein the first layer is an alloy including a first Group III material and a first Group V material, and the second layer is an alloy including a second Group III material different from the first Group III material and a second Group V material different from the first Group V material, and wherein the valence band of the first alloy is closer to the conduction band of the second alloy than it is to the valence band of the second alloy. The preferred embodiment identified In as the first Group III material, As as the first Group V material, Ga as the second Group III material, and Sb as the second Group V material.
Also, U.S. Pat. No. 4,371,884 entitled xe2x80x9cInAs-GaSb Tunnel Diodexe2x80x9d, also issued to Leo Esaki provides for a tunnel diode requiring no heavy doping, and which the process of molecular beam epitaxy can readily fabricate. The ""884 tunnel diode heterostructure comprises first and second accumulation regions of relatively lightly doped group III-V compounds specifically consisting of In1xe2x88x92xGaxAs and GaSb1xe2x88x92yAsy, where concentrations expressed in terms of x and y are preferably zero but less than 0.3, and where the improvement consists of an interface of a relatively thin layer of a quaternary compound whose constituent materials are those of the adjoining regions. This interface provides a tunneling junction as opposed to an ohmic junction between contiguous regions of InAs and GaSb.
An object of the present invention is to provide a new and useful improvement in tunneling diodes in order to expand their application to higher bandwidths, with greater dynamic range and greater sensitivity for radio frequency detection. In particular, the present invention is designed to provide a high degree of non-linearity near zero bias. This is in contrast to patents discussed above, which are designed to provide a negative resistance region for non-zero bias.
The present invention uses AlSb and AlGaSb layers to control the curvature of the current voltage (I-V) curve and current density through the device, thus decreasing the forward current while allowing the tunneling current in the negative bias direction to be relatively large and unaffected. The desirable characteristic of this design is to provide a highly nonlinear portion of the I-V curve near zero bias, which is greatly improved by the presence of the AlGaSb layers.
According to the invention there is provided a high-speed semiconductor device that exhibits an interband tunneling characteristic. The present invention comprises two semiconductor regions having mutually different compositions from one another and separated by a thin interface layer through which tunneling occurs.
The semiconductor regions exhibit gaps that are shifted in mutually opposite directions and the interface layer is amply thin to allow for electron transfer via tunneling.